1. Technical Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which is capable of repeatedly transferring an image formed on an image holding member onto a transfer sheet held on a transfer material holding member and is provided with a separation charger for activating a part of the transfer material holding member where the transfer sheet is separated after the completion of a transfer process.
2. Description of Related Art
An image forming apparatus of this kind is disclosed, for instance, in Japanese Published Unexamined Patent Application TOKKAI SHO 62-191863. A transfer material holding member is arranged to electrostatically hold a transfer sheet, and a dielectric is utilized therein. When a transfer sheet is separated from the transfer holding member after the completion of a transfer process, an aerial discharge is generated at a location where the transfer sheet is separated. The aerial discharge causes to scatter the toner of the image transferred onto the transfer sheet therefrom, and eventually, the transferred image falls into disorder.
In order to avoid such a problem, it has heretofore been arranged to actuate a separation charger on a part of the transfer material holding member where a transfer sheet is separated after the completion of a transfer process. The separation charger removes electric charge of the transfer material holding member by neutralizing the charge, for instance, by alternating corona discharge. The aerial discharge is thereby decreased to prevent the toner from dispersion.
Electric charge to be charged on the transfer material holding member varies according to the number of times an image is transferred onto a transfer sheet. For instance, even if a transfer process is simply repeated, the charge on the transfer material holding member is risen every time the transfer process is repeated.
In the known art disclosed in said publication, when an output of the transfer charger is raised every time transfer process is repeated, the electric charge to be charged is remarkably risen. However, since an output of the separation charger is set at a constant value, it happens that the electric charge is excessively or insufficiently removed according to the number of times of transfer processes.
For example, when a color image is formed by a four-color composite image forming process, the transfer material holding member is supposed to hold a transfer sheet after the completion of transfer process of a fourth color. If an output of the separation charger is set corresponding to an electric charge to be charged, the output exceeds the electric charge which is possessed by a transfer sheet and the transfer material holding member when an image is formed under three rounds of composite image forming processes. Under the circumstance, the charge of the transfer sheet is excessively removed when it is separated from the transfer material holding member by the separation charger. If the charge is excessively removed, the electrostatic adsorption force of toner to the transfer sheet is lowered.
The toner of an image transferred onto a transfer sheet tends to be scattered and causes to disorder the transferred image if the electrostatic adsorption force of the toner is lowered since the toner is affected by a mechanical shock when the transfer sheet runs into a pair of fixing rollers of a fixing device, by the electrostatic repulsive force generated by frictional charge between the fixing rollers, by the electrostatic repulsive force generated by frictional charge between the transfer sheet and a separation claw which is provided for mechanically separating the transfer sheet from the transfer material holding member, and the like.
Conversely, if an output is set corresponding to an electric charge to be possessed by a transfer material holding member after the completion of a first color transfer process, the output is insufficient for an electric charge to be possessed by the transfer material holding member when an image is formed by more than two rounds of composite image forming processes. The removal of charge can not therefore be made sufficiently when the transfer sheet is separated.
If the removal of charge can not be made sufficiently, aerial discharge is increased when a transfer sheet is separated from the transfer material holding member, and toner dispersion can not be prevented. Further, when the removal of charge is not made sufficiently, a separation claw forcibly separate a transfer sheet from the transfer material holding member to increase frictional charge between the transfer sheet since the electrostatic adsorption force of the transfer sheet and transfer material holding member is kept strong, and eventually, toner is scattered since the charge to be charged acts as an electrostatic repulsive force to the toner.